In the radio frequency spectrum, “white spaces” refer to radio frequency spectrum that was previously used by television broadcasters. One technology that is likely to benefit from white space availability is wireless networking in white spaces. In particular, wireless networks may significantly benefit from the propagation (long range) and building penetration properties of VHF and lower UHF bands compared to other unlicensed spectrum bands like the 2.4 or 5 GHz ISM bands.
In the United States, the Federal Communications Commission (FCC) is allowing unlicensed devices to opportunistically operate in white spaces. Other countries are considering similar regulations. However any such operation of an unlicensed device is (or will be) subject to meeting governmental regulations. For example, the FCC permits unlicensed devices to transmit in white spaces as long as they do not interfere with the primary licensed users of this spectrum, namely television broadcasters and wireless microphones.
To prevent any interference, spectrum sensing may be used to determine the presence of primary users. Because television receivers and wireless microphone receivers do not transmit RF signals, i.e., they are passive, unlicensed white space devices need to sense the presence of a primary user at very low thresholds, so as to ensure sufficient RF distance to the primary receiver and avoid causing interference. The FCC specifies this threshold to be −114 dBm; at least one other country is considering an even lower threshold.
However, while potentially feasible, sensing at such low thresholds is difficult from a technical perspective and is likely to require sensitive and expensive hardware. Also, sensing at such low thresholds may be inefficient in terms of energy consumption on the white space device. Current sensing technology is also prone to false alarms, as it tends to be overly conservative, which unnecessarily prevents unoccupied channels from being used and thereby wastes available white spaces.